1. Field of the Invention
The present invention relates to a method of forming a liquid crystal cell for use in a liquid crystal display (LCD) and more particularly to a method of forming a vertically aligned liquid crystal cell using a dry alignment layer.
2. Description of the Related Art
It is well known that the vertical-crystal-aligned (VA) liquid crystal display (LCD) has a good black state, and thus it has a high contrast ratio. VA-LCD has been used in both the direct view thin film transistor-liquid crystal display (TFT-LCD) panel and the liquid crystal projection light valve.
Two types of vertical alignment (VA) are commonly used. One is a xe2x80x9ctruly vertical alignmentxe2x80x9d (e.g., liquid crystal molecules are aligned 90xc2x0 to the substrate surface with zero pretilt). The other is a tilted homeotropic alignment (e.g., liquid crystal molecules are aligned to the substrate surface almost vertically with a small pretilt angle of, for example, 1xc2x0 to 10xc2x0, from the substrate normal). The truly vertical alignment is used for ridge-type, fringe field-type, or ridge and fringe field-type of multi-domain liquid crystal display structures. The tilted homeotropic alignment is used for single domain as well as some multi-domain liquid crystal display structures.
Conventionally, for the ridge type or ridge and fringe-field type of multi-domain liquid crystal structure, after ridges are formed, a homeotropic polyimide is coated to cover the surface area which includes the ridge area. Polyimide usually accumulates thicker near the ridge. This results in light leakage near the ridge. A uniform coating of the alignment material on all surfaces, whether it is on the ridge, near the ridge, or away from the ridge, is required. Silicon oxide or silicon nitride deposited film which gives vertical alignment for the negative dielectric anisotropic liquid crystal, as described below, will solve this problem.
For tilted homeotropic (vertical) alignment, two methods of alignment are usually used. A first method is using two oblique angle silicon monoxide depositions (e.g., the first oblique angle deposition is about 5xc2x0 and the second oblique angle deposition is about 30xc2x0) on the substrate surface followed by a long chain alcohol evaporation on that surface. A second method is using a mechanical rubbing treatment on the homeotropic polyimide film which is coated on the substrate.
However, the first method is complicated and costly. Also, the charge retention is usually poor, on the order of less then 80%, due to the use of the long chain alcohol material.
Furthermore, experimental results show that the stability and consistency of the pretilt angle of the second method is hard to achieve and control.
Further, it is noted that U.S. Pat. No. 5,309,264 to Lien et al. describes how one can use a fringe field produced by the electrode cutouts to make a multi-domain liquid crystal display (LCD) to improve a viewing angle of the conventional LCD.
U.S. Pat. No. 5,907,380 describes another method using fringe field and polymer wall (ridge) to make multi-domain liquid crystal display (LCD) to improve the viewing angle of the conventional LCD. For the above two multi-domain homeotropic liquid crystal displays, a homeotropic (vertically-aligned) polyimide is used as the liquid crystal alignment layer which aligns the liquid crystal vertically at the field-off state.
In view of the foregoing limitations of the conventional methods and structures, an object of the present invention is to provide a new method and structure in which a truly vertical alignment is achieved and in which a tilted vertical (homeotropic) alignment is achieved.
In a first aspect of the present invention, a liquid crystal display device (and method for forming the LCD) includes
a first substrate, a dry deposited vertical alignment layer deposited over the substrate, a second substrate coupled to the first substrate with a dry deposited vertical alignment layer deposited therebetween and forming a cell gap; and a liquid crystal material formed in the cell gap, the dry deposited vertical alignment layer deposited allowing a truly vertical alignment of molecules of the liquid crystal material such that the molecules form an angle of substantially 90xc2x0 relative to the substrate.
In a second aspect of the present invention, a liquid crystal display device (and a method for forming the same) includes a first substrate, a dry deposited vertical alignment layer deposited over the substrate, followed by an ion beam treatment, a second substrate coupled to the first substrate with the dry deposited vertical alignment layer deposited, followed by an ion beam treatment, therebetween and forming a cell gap, and a liquid crystal material formed in the cell gap, the dry deposited vertical alignment layer and subsequent treatment allowing a tilted vertical alignment of molecules of the liquid crystal material such that the molecules have a pre-tilt angle of 0.5 to 10 degrees from a substrate normal direction.
With the unique and unobvious method and structure of the invention, the present invention can achieve a truly (almost 90xc2x0) vertical alignment, and also can achieve a reliable tilted homeotropic liquid crystal alignment with high charge retention.
In a third aspect, a liquid crystal cell (and method for forming the same) includes a first substrate, a pixel electrode disposed above the first substrate, a second substrate, a transparent conductive electrode disposed below the second substrate, a liquid crystal material disposed between the pixel electrode and the transparent conductive electrode, a polymer ridge layer built on the conductive electrode,a dry deposited vertical alignment layer deposited on the first and second substrate. The polymer ridge controls a tilt direction of the liquid crystal material.
In the third aspect, a new structure for a multi-domain homeotropic liquid crystal display is provided. In this new structure, a dry alignment layer (e.g., a dry-deposited film as an alignment layer and without a rubbing treatment) in combination of ridges and/or pixel slit cutouts is used to control the liquid crystal (LC) molecule tilt direction in the pixel to improve the display viewing angle.
This method can be used for both multi-domain LCD""s and single-domain LCD""s, and is applicable to both high density (e.g., having about 130 dots per inch (DPI) to about 250 DPI) and low density (e.g., having about 30 to less than 130 DPI) liquid crystal displays. It can be used for direct view thin-film-transistor driving liquid crystal displays (TFT-LCD), and it can also be used for liquid crystal light valves (e.g., both transmissive and reflective ones).
Another distinct advantage of this method of inducing alignment, is that the alignment layer in this case may also serve as a passivation layer for the TFT array. It is known that bare metal surfaces exposed directly to PI alignment layers and LC materials can cause instabilities in the final assembled panel over time. For this reason, it is common practice to encapsulate the entire active array using a final layer, almost invariably PECVD silicon nitride. Since the present inventors have established that SiNx, SiOx and any silicon oxynitride SiOxNy having a composition bounded by these two will provide vertical alignment, it is clear that the two distinct steps of array passivation and alignment layer deposition can be combined into one single cost-saving step.